This invention relates to ohmic contacts using tin and more particularly to ohmic contacts on N type gallium arsenide, gallium phosphide, aluminum gallium arsenide or aluminum gallium phosphide.
One of the major drawbacks in forming a tin type ohmic contact on N type gallium arsenide, aluminum gallium arsenide, gallium phosphide and aluminum gallium phosphide is that the tin tends to ball up on the semiconductor surface when heat is applied to drive the tin into the semiconductor material. This drawback is particularly evident in gallium arsenide and aluminum gallium arsenide electroluminescent devices where a uniform current distribution is desired. The tin has a high surface tension and a low wetting ability on gallium arsenide, gallium phosphide, aluminum gallium arsenide and aluminum gallium phosphide which prevents the formation of a uniform film and as a result prevents the formation of a good uniform electrical contact. The current flows from the ohmic contact into the device only through the balls of tin and therefore through only a small portion of the surface of the semiconductor material.
It is well known that ohmic contacts can be formed of various alloys. Included among these are alloys of tin and gold. However, the prior art does not teach a method of alloying gold and tin on these semiconductor materials in a manner which avoids the balling up on the surface.